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<html lang="en">

<head>
    <meta charset="UTF-8">
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    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        var num = 1;

        function demo() {
            console.log(num); // undefined

            function demoSon() {
                console.log(num); //2
                num = 3;
                console.log(num); //3
            }
            var num = 2
            demoSon();
        }
        demo();
    </script>
</body>

</html>